The largest hurdle in safe operation of a flight display is to ensure the extreme improbability of displaying “false or misleading information” to the pilot. False or misleading information may include information that may be accurate or correct as of a time t0, but no longer accurate or correct as of some subsequent time t1 (due to, for example, continuous changes in the perspective and position of an aircraft as it proceeds through space). This particular type of false or misleading information may result from a “stuck” or “frozen” display. Worse yet, the pilot may not know that a stuck or frozen display is providing false or misleading information, instinctually relying on the display contents as though they reflected reality although this may no longer be the case.
For example, an active-matrix liquid-crystal display (AM-LCD) may hold information on a screen or display surface. Images are “drawn” to the display surface by charging capacitors to a drive voltage proportional to the brightness of each individual display element (e.g., pixel, subpixel). These capacitors may include: (1) the electrodes used to directly control the state of an LCD subpixel; (2) component parts of the drive circuitry for controlling the current drive of an organic light emitting diode (OLED) sub-pixel; or (3) component parts of the drive circuitry for other matrix-driven display types. The display surface may correspond to a two-dimensional M×N array of such capacitors (e.g., M rows of N capacitors each), enabled one row at a time starting with the topmost row. When a row of capacitors is charged to the appropriate drive voltages, the row may be isolated by deactivating its controlling transistors and activating the transistors corresponding to the next row, thereby charging the next row of capacitors. In this way, each individual row may be refreshed until the entire display surface has been freshly drawn to, at which point the process is repeated starting with the topmost row. The display surface may thus be refreshed may times per second (e.g., at a predetermined “refresh rate” such as 60 Hz) such that the human eye perceives the display contents as continually dynamic (provided the refresh rate is faster than the human flicker frequency threshold).
If the controlling circuitry becomes disabled, the capacitor charging process may stop, “freezing” each row of capacitors in their last charged state and the display as a whole in its last commanded condition. It may be possible to monitor the display system in order to detect failed circuitry, but sensor-based monitoring is at once expensive, bulky, and cumbersome. Consequently, while such monitoring may be found in some types of customized display systems, it is rare in cost-effective commercial off the shelf (COTS) displays. Furthermore, even a sophisticated sensor-based monitoring system may cover only a limited portion of a display surface, or a limited number of possible failure types. Absent any sort of active monitor for detecting failed drive circuitry, a last commanded image will remain onscreen. However, the pilot may not perceive the display as stuck or frozen until capacitor voltages deteriorate or leak to a significant enough degree to degrade image quality and render the stuck condition obvious to the naked eye. Prior generations of AM-LCD displays, for example, may be associated with higher voltages required to drive a normally white display to black levels. As degraded black performance more rapidly affects the uniformity and contrast ratio of a display, it is highly visible—rendering a frozen LCD obviously invalid within a few seconds.
However, advances in AM-LCD technology may complicate, rather than alleviate, this problem. Contemporary AM-LCD systems tend to use a normally black approach rather than a normally white approach. Similarly, such AM-LCD systems may have better and less leaky transistors, and use purer liquid crystal fluids less subject to capacitor leakage. As a result, the black backgrounds of frozen displays may linger far longer while only bright portions degrade due to leakage. Further, as this leakage is far slower, a frozen AM-LCD may appear unaffected for a minute or two before any degradation is perceived—which may lead to extremely unsafe flight conditions if the pilot instinctually relies on such outdated visual intelligence without cause to question it.